Three months ago a new clothes shop appeared in Soho in central London. Just off Carnaby Street, opposite a designer boutique with a T-shirt in the window saying "Fuck Off" in delicately stitched capitals, the new Barbour store looks a little out of place at first glance. A huge faded union flag is draped inside the doorway as if in a regimental chapel. Old metal jerrycans, seemingly straight from a farmyard, stand under the clothes rails. And hanging in heavy rows are Barbour's trademark jackets, tent-like and stiff, in muddy country colours, and with antiquated, upper-class names such as the Bedale and the Beaufort. It is hard to see the shop lasting.

Yet early on a raw weekday morning, with some neighbouring shops not even open yet, there is already a steady flow of Barbour customers. Some are the sort of people you might expect: middle-aged, prosperously dressed, pinkish Horse and Hound complexions. But others are quite different.

A man of about 30 with fashionably rolled-up jeans, angular hair and a manbag strides in. He flicks through the rails, then lingers over a rack of quilted country jackets that look like something the Queen would wear. "I think he wants that kind of old-style, boxy one," murmurs one sales assistant to another. The man lingers for several more minutes, but doesn't find quite what he is looking for. Yet he can probably live with the disappointment. As he passes me on his way out, I realise he is already wearing a Barbour.

"We opened in September and we have already had to shut for a week to re-stock," says a sales assistant. "The heritage styles – the young people want them. They wear them really fitted. Small sizes." She gives a faintly incredulous look: "It's funny to see the Barbour become a fashion item. I always associate them with hunting and fishing."

Perhaps not for much longer. Over the last few years, in trendy parts of London, at music festivals, and among clothes-horse celebrities, Barbours have become so ubiquitous that there is even a half-mocking nickname for the look, referencing the London borough where it is commonest: "Hackney farmer". The boom is even affecting sensible old John Lewis: this year, national sales of Barbours there are up more than 80%.

And the Barbour craze is only part of a much broader new appetite for products with posh associations. Brogues and Oxfords, the more clumpy and traditional-looking the better, have become the shoes of choice for many cool young men. Hunter wellies, once something for squelchy point-to-points, are now what Kate Moss and her fashion disciples wear to Glastonbury, and are currently on sale at Jigsaw. Tweed has become hip. So have faintly caddish moustaches and even – according to the high-street chain Vision Express – monocles. "There was recently a trend for waistcoats, a very young man's trend," says the fashion journalist Charlie Porter. "There is a thing for bow ties right now. There's no way a bow tie can be anything but posh." The Official Sloane Ranger Handbook – published more than a quarter of a century ago, the last time there was a vogue for upper-class style – lists the key items in the male Sloane wardrobe: "the thick, woolly Action Man sweater . . . the blazer . . . trousers worn a bit short . . . the cardigan." Over the last few years, British men's fashion magazines have featured little else.

On television, too, poshness is enjoying a boom. Hugh Fearnley- Whittingstall, Kirstie Allsopp, Thomasina Miers, Valentine Warner – their unapologetic private school vowels, patrician brusqueness or charm and, in some cases, aristocratic backgrounds (Allsopp is actually The Honourable Kirstie Allsopp) – have become assets in what is usually considered the most populist of media, until recently dominated by self-made Britons and regional accents. "Kirstie and Hugh are posh. They know that. We know that," says Andrew Jackson, who commissions their shows for Channel 4. "Maybe in the past they would have hidden it. Television used to be about the middle class and the working class. But over the past two or three years [posh] presenters have become less ashamed."

Jackson also oversees a successful reality programme about restoring grand properties called Country House Rescue. "It's a world that television ran away from for a long time, but the viewers are quite interested. Maybe there's a slightly aspirational side to it. Viewers don't want to live in a country house exactly, but maybe they'd like to live a bit like someone who has one." There is currently a trend in design-conscious London homes for hanging faux-baronial pairs of antlers.

There have been other straws in the wind: surprise hit books by toffs, such as (Sir William Robert) Ferdinand Mount's 2008 memoir Cold Cream; once-geezerish London hellraisers such as Damien Hirst and Alex James reinventing themselves as country landowners; successful models with grand backgrounds such as Honor Fraser and Jasmine Guinness and Jodie Kidd. Membership of the British Association for Shooting and Conservation has grown by almost a sixth in the last decade. Despite the recession, pupil numbers at the grandest private schools rose this term, according to their trade body, the Headmaster and Headmistresses' Conference. In 2007, the social commentator Peter York, who co-wrote the Sloane Ranger Handbook with the journalist Ann Barr, published a sequel, Cooler, Faster, More Expensive: The Return of the Sloane Ranger. In it he wrote, "It's cool to be smart, again."

Meanwhile, from politics has come the least subtle signal of all that poshness is back in the mainstream of British life: the largely untroubled rise of the most patrician generation of British politicians for half a century – David Cameron, George Osborne, Boris Johnson, Zac Goldsmith. So far, despite increasingly open class attacks on them by senior Labour figures from Gordon Brown downwards, despite the anti-elite atmosphere created by the recession, there is little strong evidence that the backgrounds of these posh Tories are decisively counting against them.

Connecting cultural and consumer trends with political ones is an inexact science; but the return of poshness, in all its manifestations, probably has the same basic root. Most Britons now consider the upper classes relatively harmless. "Posh people are quite associated with environmentalism, with food and cooking," says cultural historian Joe Moran. "They are not the folk devils of our time: the bankers, the globalisers." Porter says that traditional upper-class clothes have long been superseded as symbols of power: "Chinos, the super-boring suit – what people wear to EU and G20 meetings – that's elite dress." As the ruling-class connotations of posh styles have faded, he argues, so these styles have been gradually decontaminated for everyone else.

Almost 20 years ago, in The Decline & Fall of the British Aristocracy, the historian David Cannadine convincingly described how the British upper classes were steadily squeezed by the 20th-century's economic, political and social forces until they retained "only an infinitessimal part" of their former dominance. Yet he also noted the surprising resilience of their cultural influence. "In many areas of British life," he wrote, "the aristocratic tone lingers on." York explains this in bald commercial terms: "These [posh] people, having owned and ruled half the world, have got some nifty stuff, so the modern consumer thinks, 'Let's not ignore it on politically symbolic grounds.'" Porter cites the long- running advertising campaign for Burberry in glossy magazines: "They have sons of the aristocracy jumping around models. The aristocrats tend to have good cheekbones, so they look the part. And the country house setting sells well in America."

Abroad, upper-class Britishness has long been consumed as pure style. Italian football hooligans wear Barbours. But in Britain poshness seems to appeal across social classes more intermittently, and in particular political circumstances. In the mid-70s, when York first started noticing Sloane Rangers, and the British fashion label Mulberry successfully started selling the "English hunting, shooting and fishing look", the country was beginning to move to the right after the egalitarianism of the postwar decades. The hugely popular television version of Evelyn Waugh's nostalgic country house novel Brideshead Revisited (adapted again, significantly, for the cinema last year) started filming weeks after Margaret Thatcher was elected in 1979. As her influence on Britain strengthened in the early 80s, Moran remembers a fully fledged "return of the posh – young fogies, Land Rovers – a certain rural chic". Fashion designer Vivienne Westwood, who had been making lurid punk outfits a few years before, developed an interest in tweed. Barbours were considered cool, remembers the sales assistant in its new Soho shop, with a knowing expression, "for about 18 months".

This poshness craze in the 70s and 80s was partly an explicit reaction against the Labour-governed Britain that had gone before, with its flat-capped union barons and proletarian flavour. Behind the present shift to poshness, a similar process has been under way. Britain under New Labour may have been dominated by the middle class rather than working class, but it has had enough demotic, sometimes toff-baiting aspects – from Tony Blair's glottal stops to the hunting ban to the huge yobby hits by Oasis – to provoke a counter-revolution. In a 2003 Sunday Times interview, Johnnie Boden, the Old Etonian mail-order magnate and, famously, supplier of casual upper-class clothes to David Cameron, summarised the Boden aesthetic as follows: "It's not Blairite, it's not Britpop . . . We sell to people who don't buy into that culture."

For a long time, as New Labour won election after election, such keepers of the Sloane flame, however individually successful, seemed more like a rearguard rather than a vanguard. In 2002 the Countryside Alliance flooded London with 400,000 field sports supporters, but their massed Barbours looked old-fashioned not fashionable, and hunting was still banned. Yet as early as the late 90s a taste for the patrician had been quietly beginning to re-emerge: for Farrow & Ball's country house paint colours; for the gentleman's club cuisine of the Two Fat Ladies (aka Clarissa Dickson-Wright and Jennifer Paterson); for the upper-class bohemian aesthetic marketed by the mail-order company Toast; for the Prince William and Harry-style clothes of the new "University Outfitters" chain Jack Wills.

These products were increasingly sold to the expanding middle class as well as the old upper classes, often by a new breed of Sloane entrepreneurs, personified by Boden. "No longer iffy about money, this [type of] Sloane is not only up to competing in a merit-ocracy," wrote York in The Return of the Sloane Ranger, "but chances are he'll have access to cash and contacts . . . Unassailed by doubt and with his unstoppable sense of entitlement, he's out to rule the world."

Other recent trends have worked in favour of the new poshness. Environmentalism, and the growing demand for the locally sourced or hand-crafted often go more with the grain of gentlemanly, landed capitalism than with the urban corporate version. The revived consumer interest in Britishness, from hearty food to "heritage" brands to blustery seaside holidays, has seen many middle-class people, consciously or not, rediscovering tastes the upper classes had never completely lost. Even the bestseller lists have taken on a posh tinge: The Dangerous Book for Boys, with its jolly talk of knots and rugby and conkers; the magical boarding school at the heart of the Harry Potter books.

Meanwhile, the renewed acceptability of prejudice against parts of the working class – "chavs" – has made social hierarchy in general seem more natural again. And finally, people have simply grown tired of the sometimes blandly managerial, sometimes matey ruling style of New Labour: "The mock-demotic aspect of Blair culture – people are disillusioned with that now," says York. "People came to feel it was a hypocritical culture. They came to have a preference for the clearly elitist – for Boris."

How long will the appetite for poshness last? Some of its beneficiaries are well aware that their popularity may be precarious. Andrew Jackson recalls, "Hugh [Fearnley-Whittingstall] always says to me, 'If I'm onscreen teaching the viewer something, I'm 'Hugh Fearnley-Whittingstall'. If I'm not teaching them anything when I'm onscreen, I'm 'a posh twat.'" John Curtice of the university of Strathclyde, a leading authority on British political attitudes, says the current crop of patrician Conservative politicians are similarly vulnerable: "Cameron's poshness will get used against him [effectively] if he's unpopular for other reasons. George Osborne is not as engaging as Cameron, so his poshness is already held against him."

In British life and politics, class is always a bit of an unexploded bomb. The decline of the aristocracy has not been complete enough to defuse it: as York points out, "A lot of people who own the land are the people who [always] used to own the land." Next to comparable countries, Britain has low social mobility, high levels of inequality, and an education system unusually segregated between the privately educated and the rest; in this context, class privilege and its signifiers have particular potential to stir resentment. Even at the Barbour shop in Soho, it is striking that the strenuously rural decor omits any images of people wearing the clothes for hunting or shooting.

The poshness fad has limits. Moran, who teaches at Liverpool John Moores university, says that, like enthusiasm for Cameron, it is much less evident in the north of England (although the Barbour craze has taken root in Manchester). Porter notes that the trend for Barbours and brogues has not spread to working-class Britons. And the Conservative party, usually sure-footed these days when it comes to managing its image, has seemed jittery and clumsy over the grand backgrounds of many of its leading figures: omitting the private schools they attended from personal biographies on its website, suggesting to some Tory parliamentary candidates that they shorten their double-barrelled surnames, and showing unease over the now-infamous 1987 photo of Cameron and Johnson and their Bullingdon Club chums at Oxford.

But Curtice thinks that class warriors should not be licking their lips quite yet. "Because the Labour party is no longer class-based, it finds it harder to pursue [toff-bashing] when it returns to it." Blair himself, Curtice points out, "had a very privileged education" at the leading Scottish private school Fettes; you could even argue that Blair's gentrification of Labour was the beginning of the whole current patrician phase in British politics.

Yet a Cameron government, committed to cutting inheritance tax, committed to a free vote on hunting in the Commons – which would likely repeal the ban – would certainly test modern Britain's appetite for poshness. Wearing country clothes may feel different for urban hipsters once the Tory shires are politically dominant again rather than politically powerless.

For now, the fashion for Tory chic goes on. Round the corner from the Soho Barbour shop, there is another, even newer and more fashionable shop selling tweed suits, and T-shirts printed with portraits of political leaders. The Thatcher print is particularly commanding. How is it selling? "Better than any of the others," says the store manager.